Did Magmaw on 25 man last night and I kept dying to a melee hit from him, happened maybe 10 times. It always occurred as he came back up after the head phase. I wasn't top on threat though I may have snuck into second (after the MT). He just seemed to look at me , hit me, I'd die then the fight would continue properly with MT at top of Omen and all normal.

Having a read around, it seems this is commonplace. The info on why it happens and how to avoid it all seems a bit speculative, contradictory and less than concrete though...moving away just before head phase ends seems to be the safest approach

It's not a bug. After the burn phase (head phase) he resets aggro when coming back up again. As a melee dps you HAVE to let the tank pickup the boss before going back in.

What I do is step out of melee hit box when the boss rises again and let the tank pick up, then I go back in...the 1-2 seconds of dps time lost is well worth eliminating the untimely death from picking up the boss before the tank does.

Since we have melee on the left side of him and the head drops you on the right side of him (and I am one of the chainers) I just take a circuitous (as in a circle) path back to his left side and only engage once he is back on a tank. The lost dps from a few seconds of waiting is less than the lost dps from dying...

If you stand behind him before the head phase, you don't want to be in that same spot in the head phase. You want to be over near the spike somewhere. And just fyi when you hit the head its called "exposed back of magmaws head" so you will always be behind it no matter where you stand. It does some kind of swipe it usually hits for like 90k in 10m reg. But it is listed as a melee hit.

This is my assumption, I have not had the problem even once since I started standing near the spike. It would happen almost every attempt when I stood behind him.